I'm sorry. This is tough. My only answer would be this requires constant vigilance. The minute the older child gets aggressive with anyone they are told in a firm voice with a serious, unhappy face that we do not bite/hit/pinch. I then set the older child down away from me (and the older child is now crying) and pick up the baby/victim and sooth them.Â
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When my younger daughter was entering this stage and being aggresive with her older brother I had to be watching her constantly. The bite always followed a pattern. Older brother would have something or have take something from younger sister and she would open her mouth and head in for the bite to get it back. Luckily, the bite only happened once before I figured out the pattern. I watched her like a hawk for the weeks after that. Everytime she opened her mouth and headed in, I immediately, made a loud "EHHHHH!!!" sound. (I don't like saying no to kids all the time.) It would startle her into stopping the action and everyone would look at me and I say in my firm voice, "Remember, we don't bite." Usually, she would cry and I would snuggle her and hug her and tell her she's a good girl, but she needs to remember we don't bite people, it hurts them. If brother takes something from you, you come tell mommy and we'll figure it out.Â
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Now, at 1, I'm sure she didn't understand all that. But, I know she figured it out. First, I would catch her going in for the bite then changing her mind on her own and then just complaining to me instead. It didn't take too long, a couple of weeks and there has been no back slide.Â
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Also, I have to add, that I don't think hurting a child to teach him not to hurt others is effective. (Please don't say that when you "flick" him or smack him on the bottom it doesn't hurt him.  You're not giving him a hug, your intention by hitting him is to cause him pain. If it tickled, he'd laugh.)Â